sotropic. By drawing the materials at slightly elevated temperatures, the modulus (stiffness) may be increased from 59 to 124 GPA, the values for Kevlar and Kevlar 49. Kevlart melts at over 500 degrees Celsius, is exceptionally high in strength with a tenacity more than twice that of high strength nylon or polyester, and a very high modulus. The structures of these fibers are highly crystalline. Kevlar's density of 1.44 g/cm cubed is among the highest for organics, while molecular weight of its PPTA (pphenyleneterephthalamide) molecules are considered rather low. Although some details of the crystal structure of Kevlar have been uncovered by x-rays, a comprehensive model does not exist. Recent studies of crystallites in Kevlar show that the crystallites are slightly misaligned due to the fiber axis, but can be oriented by a tensile load. Since Kevlar has low compressive strength, it can knot from compressive failure due to collapsed zones. Because Kevlar has a fibrillar structure, it splits axially over distant lengths, unlike most other high strength fibers that break straight across. To fracture the material, a crack must traverse over different planes. With fibrillar fracture, it makes cutting by machine tools or projectiles difficult. Thus making it excellent material to produce bullet proof vests and helmets. Unfortunately since Kevlar is a polymer held together by an amide bond, it is degraded by UV (ultraviolet) rays. WHY THE Z PLANE IS THE WAY TO GO? With an X, Y plane, the strength of the Kevlug relies on the breaking resistance of the fiber. Breaking strength for Kevlar is low because Kevlar filaments are small, thus making the X, Y plane a bad choice. With the Z plane, the polymers that would have been scraped away are left intact, making wear occur at a slower rate. With a Z plane, one wears the fiber out slower (something along the lines of molecule by molecule), like an eraser head would by erasing something, instead of te...
